Electrophotographic printing is achieved by a process which includes the step of uniformly charging a photoconductive surface and exposing the uniformly charged photoconductive surface to a light source so as to discharge selected portions of the photoconductive surface thereby forming a latent electrostatic image. The latent electrostatic image is developed by applying toner to the discharged portions of the photoconductive member. The applied toner is then transferred to a medium such as paper on which the copy of the image is to be formed. The term electrophotographic printing as used herein is intended to encompass electrophotographic printing and electrophotographic copying.
Such apparatus includes a developer unit operative to deliver toner to the photoconductive member. Typically, the toner is stored in a hopper where it is mixed with a suitable carrier. Typically the carrier comprises iron or other metal particles. When mixed with the carrier, the toner acquires a suitable electrostatic charge so that it may easily be transferred to the photoconductive belt to develop the latent electrostatic image formed thereon.
The developer unit requires considerable maintenance to insure that the proper ratio of toner to carrier is present to produce satisfactory copies. If the toner to carrier ratio is too high too much toner is deposited on the photoconductive surface and the resulting printed copies will smear. If the toner to carrier ratio is too low too little toner is deposited on the photoconductive belt and the resulting printed copies will be too light. Consequently, easy access to the developer unit is required to allow maintenance of the developer unit.
In some electrophotographic apparatus, the entire developer unit is removable. One problem with such removable developer units is that the spacings between the developer unit and the photoconductive surface should be maintained within certain tolerances. Such tolerances are difficult to maintain when the developer unit is constantly removed. Accordingly, relatively expensive alignment mechanism are required to maintain the appropriate spacing between the developer unit and the photoconductive surface while simultaneously permitting removal of the developer unit. U.S. Pat. No. 4,203,386, issued to Blochl et al. May 20, 1980 describes one such alignment mechanism.